A form overlay type document printing apparatus is described in each of Japanese Patent Applications No. 10143/81, and 71081/82. The printing apparatus disclosed in each of these laid-open application includes a printing data buffer and a form buffer and is advantageous in that when the same data are repeatedly printed, if form data have been transmitted once from a host computer and stored in the form buffer, it is sufficient thereafter to transmit only printing data until the form is changed. That is, it is not necessary to transmit a combination of printing data and form data from the computer every time. Also, a printing pattern and a form pattern can be generated in the printing apparatus by transmitting simple codes only. This simplifies the processing at the host computer side and reduces the amount of data that must be transmitted.
In the prior art, however, there is a problem that in the case where a document comprising a number of sheets is to be produced with the same form and with some data which varies with every sheet and some data which is common to every sheet. This situation requires the combination of the varying or unique data and the common data to be transmitted for every sheet from the computer. As a result the time required for data transmission is prolonged and the efficiency of the transmission line is reduced.